1. Field of the Invention
The present invention relates generally to charge-coupled devices (CCDs), and more particularly pertains to CCD imaging devices with a three dimensional CCD cell and isolation trench structure to achieve highly effective light sensing and charge storage areas with a small cell layout area and also with highly efficient transfer of charges from cell to cell within the CCD device.
This patent application is related technically to U.S. Pat. No. 5,173,756, filed Jan. 5, 1990, and commonly assigned to the same assignee, but is quite distinct therefrom. While both the present application and U.S. Pat. No. 5,173,756 disclose and utilize charge coupled devices with trench structures, in U.S. Pat. No. 5,173,756 the trenches are utilized as electrodes, and are oriented perpendicular to the direction of charge transfer. In contrast thereto, the trenches of the present application are utilized for isolation between adjacent CCD cells, are oriented in the direction of charge transfer, and are oriented orthogonally relative to the trenches of U.S. Pat. No. 5,173,756. Utilizing the trenches for isolation between adjacent CCD cells and arranging the trenches in the direction of charge transfer as disclosed and taught herein results in several distinct advantages over the prior art. For example, the charges are transferred laterally from cell to cell along the sidewalls of the trench, rather than from trench to surface up and down the sidewalls, such as in U.S. Pat. No. 5,173,756, which results in a more efficient charge transfer efficiency (CTE). Considering that a typical CCD device might comprise three thousand cells in each column, a more efficient transfer of charges from cell to cell results in a CCD device having a higher modulation transfer function (MTF), a higher signal to noise ratio and other advantages discussed more specifically hereinbelow.
2. Description of the Prior Art
CCD devices have become increasingly important components of semiconductor technology. A CCD is a dynamic device which transports charges along a given predetermined path under the control of timed clock signals or pulses. CCD devices can be used in a variety of applications including memory applications, logic function applications, signal processing applications, image acquisition and image processing applications.
A conventional CCD structure is fabricated with an overlapping electrode gate structure. The CCD includes alternating polycrystalline silicon electrodes and polysilicon electrodes, with a layer of silicon dioxide interposed between the electrodes and a silicon substrate. The charge is stored and transferred along the semiconductor oxide interface, which is a two dimensional surface parallel to the substrate.
For high resolution imagers employing a CCD, the CCD cell size, in terms of layout area, has to be scaled down to achieve a high pixel count per unit imaging area in order to keep the total chip area at a manageable size with a reasonable yield.. CCDs currently being fabricated have a two dimensional configuration wherein the charge storage capacity and the light sensitive imaging area are directly proportional to the layout area of the CCD cell on the surface of the silicon substrate. Therefore, a high resolution CCD imager is achieved at the expense of a smaller total charge storage capacity and light sensitivity because of the down scaling of the CCD cell layout area. Furthermore, the resolution (measured by the figure of merit of, for example, the modulation transfer function, MTF) of the down-scaled CCD imager is degraded because of the two dimensional nature of the CCD potential profile and the increased proximity of neighboring CCD cells. Photo-generated charges are more easily diffused into neighboring CCD cells in small CCD cells, and result in a smaller MTF. Present techniques for minimizing such diffusion utilize recessed oxide (ROX) and local oxidation (LOCOS) technologies.
U.S. Pat. No. 4,234,887 to W. B. Vanderslice, Jr. discloses a technique for increasing the charge storage capacity per unit area of a CCD imaging device. The disclosed device includes a plurality of parallel V shaped grooves etched into a semiconductor substrate. The grooves are electrically isolated from each other by intervening strips of relatively thick field oxide. A plurality of isolated electrodes are provided orthogonally to the etched recesses such that a shift register structure is formed which extends along the length of each recess. In operation, the packets of charge are transferred by potential wells formed by the electrodes entirely within the grooves. The use of the sidewalls of the grooves to transfer charge results in a certain amount of area reduction. However, the V-grooved CCD has a fixed cross sectional aspect ratio, as any increase of storage capacity is accompanied by an increase of the planar device area.